Conventionally, nickel plating has been widely used as a surface treatment for ornamentation and costumery. However, it has been pointed out that the nickel plating has a problem of nickel allergy that causes skin eruption or inflammation to an individual who puts on an ornament having a nickel coating so that an alternative technology has been demanded. Also, tin-lead alloy plating containing lead has conventionally been widely used as a surface treatment for electronic parts. In consideration of the harmfulness of the lead contained therein to human bodies and the environment, new plating without the use of lead has been demanded.
Under the circumstances, copper-tin alloy plating has been reviewed in recent years.
Plating baths for use in industrial copper-tin alloy plating are mostly those containing cyanic ions such as a cyanogen-stannic acid bath and a cyanogen-pyrophosphoric acid bath. Due to a severe drainage treatment regulation, disposal of waste from those baths is costly. There is also a problem from the standpoint of operation in a safe environment. Therefore, a copper-tin alloy plating bath containing no cyanic ion (hereinafter, referred to simply as “cyanogen-free”) is required.
Under the circumstances, the following pyrophosphoric acid baths have been proposed as cyanogen-free copper-tin alloy plating baths.
In JP 10-102278 A, there is proposed, as a cyanogen-free pyrophosphoric acid bath, a pyrophosphoric acid bath for use in copper-tin alloy plating that contains a reaction product of an amine derivative and an epihalohydrin in a 1:1 mole ratio and an aldehyde derivative and, when necessary, further uses a surface tension adjusting agent. Also, in JP 2001-295092 A (U.S. Pat. No. 6,416,571 B), there is proposed, as a cyanogen-free pyrophosphoric acid bath, a pyrophosphoric acid bath for use in copper-tin alloy plating that contains a reaction product of an amine derivative and an epihalohydrin in a 1:1 mole ratio and a cationic surfactant and, when necessary, further uses a surface tension adjusting agent and a bath stabilizer.
Conventionally, barrel plating has been used as a mass plating treatment method for small parts that are small in size and have no engagement hole. When barrel plating is performed on an industrial scale (in several kilograms (kg) or more), the pyrophosphoric acid baths used in the prior art give plated products with appearances (color tone, gloss, etc.) not completely uniform even for products that have been plated in the same barrel in the same plating chance and there have been problems that defective products caused by bad appearance are generated in ratios on the order of 20 to 50%. Also, removal of the generated defective products must be coped with by sheer numbers and the removed defective products must be replated, so that much labor and cost have been needed.
Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art, to provide a pyrophosphoric acid bath for use in cyanogen-free copper-tin alloy plating that can be utilized on an industrial scale, in particular, a cyanogen-free pyrophosphoric acid bath for use in copper-tin alloy plating capable of performing uniform treatment and that exhibits a low defective product generation rate (hereinafter, in some cases referred to simply as “rejection rate”) even in those applications where the state of current application is incessantly changing between a high current density state and a low current density state, as in the case of a barrel plating method, and a copper-tin alloy coating which can be obtained by using the pyrophosphoric acid bath.